1. Field of the Invention
The invention relates to purification of germanium bodies and somewhat more particularly to a process of purifying germanium bodies via a gettering technique.
2. Prior Art
In semiconductor technology, particularly in radiation measurement technology, germanium in its purest form is required. Any impurity having a doping effect on germanium must be at a minimum within any germanium body used in such technology. In this regard, the desired degree of purity is a germanium body having less than 1 impurity atom per a minimum of 10.sup.10 and more preferably per 10.sup.11 or 10.sup.12 germanium atoms. A suitable purification process yielding such results is the zone melt process. This process is based on the fact that when a melt of germanium solidifies, any impurity atoms within the molten germanium pass into the solid state either earlier or later than the pure germanium atoms. The ratio of the amount of impurities in the solidified material to the amount of impurities in the melt is defined as the segregation constant of the impurity involved. During a zone melt process wherein a melt zone passes through a semiconductor body having, for example, the form of a rod, impurities which have a segregation constant greater than 1 accumulate in the rod section which has resolidified after passage of the melt zone while impurities which have a segregation constant less than 1 accumulate within the melt zone. By repeatedly passing a melt zone through a semiconductor rod one is able to collect or segregate impurities having a segregation constant greater than 1 at one end of the rod and collect impurities having a segregation constant less than 1 at the opposite end of the rod. After a desired degree of purity is achieved, the rod ends, which now contain a concentration of impurities, may be severed from the main body of the rod. In order to achieve a sufficiently high degree of purity, the above process steps may be repeated a plurality of times.
During purification of germanium, the removal of aluminum and boron impurities are particularly troublesome because these elements do not appear to obey the above discussed general principles. Thus, a germanium body which was zone purified in this manner within a quartz crucible has a generally uniform high aluminum and boron concentration along the zone purified portions of the germanium body, despite the fact that aluminum has a segregation constant of 0.073 and boron has a segregation constant of 17, both of which are far removed from 1. This abnormal behavior may be explained by the fact that impurities form complex compounds, for example, with oxygen, and these compounds have a completely different segregation constant than atomic aluminum and/or boron. It is known that aluminum-oxygen complex compound formation may be prevented by providing a high concentration (at least about 10.sup.17 atoms per cubic centimeter) of silicon within the germanium body being purified (compare a lecture given by E. E. Haller, W. L. Hansen, G. S. Hubbard and S. S. Goulding at The Nuclear Science Symposium on Nov. 19-21, 1975 in San Francisco, California). However, for most types of applications, germanium must be substantially free of silicon. Thus, intermixing a high concentration of silicon into a germanium body to be zone purified in order to prevent formation of complex aluminum and/or boron compounds is not practical, nor typically utilized.